The invention relates to a method for controlling a high pressure gas injection internal combustion engine.
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment, e.g. working machines. The invention can also be applied to cars. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle type.
High Pressure Gas injection (HPGI) internal combustion engines have been the subject of increasing interest and use for some time. The HPGI technology is also known as the High Pressure Direct Injection (HPDI) technology. It allows natural gas engines to operate at the same efficiency and power as modern heavy-duty diesel engines, but with a better fuel efficiency and reduced greenhouse gas emissions depending on the composition of the gaseous fuel used. The HPGI heavy duty gas engine technology is based on direct in-cylinder injection of gaseous fuel providing conditions for mixing limited combustion, or mixing controlled combustion, similar to the process in conventional diesel engines. The gas is supplied using a special high pressure gas injector. Additional pilot quantities of diesel fuel are injected in order to accomplish ignition.
In HPGI engines there are usually requirements to dispense of high pressure gas, e.g. during a rapid pressure, decrease in the injection system due to a decrease in the engine load, or during an engine stoppage. A need to vent boil-off gas from the liquefied gaseous fuel storage is another common reason for disposing of gas. Such disposal will create an environmental disturbance, since it involves emitting unburned hydrocarbons into the atmosphere. In case of methane gas such emissions also cause wanted contributions to global warming.
CA2868338A1 suggests, for an internal combustion engine with direct gas injection, capturing vented gaseous fuel, storing it in an accumulator, and reintroducing it for engine combustion at a later time. The gaseous fuel from the accumulator is introduced upstream of an air intake compressor and a gas and air premix is thereby added in the cycles in the cylinders before the regular direct gas injection. A problem with this solution is that a large portion, at least 30%, of the captured fuel may not be combusted in the cylinders, and therefore, the relief provided from an environmental point of view is relatively marginal.
It is desirable to reduce emissions from fuel systems of internal combustion engines with high pressure gas injection.
In accordance with an aspect of the invention, a method of controlling a high pressure gas injection internal combustion engine is provided, the method comprising
injecting, in a first combustion mode, by means of a first gas injection system, a first gaseous fuel into a cylinder of the engine, and
accumulating in a container of a second gas injection system excess gaseous fuel from the first fuel system,
characterized by shifting, in the cylinder, from the first combustion mode to a second combustion mode comprising
determining a value of an air flow related parameter indicative of an air mass flow into the cylinder,
determining based on the determined air flow related parameter value a value of a fuel flow related parameter indicative of a mass flow of the excess gaseous fuel, and
supplying from the container, in accordance with the determined fuel flow related parameter value, the excess gaseous fuel to provide a premix of air and the excess gaseous fuel to the cylinder.
The invention provides for avoiding release of the excess gaseous fuel into the atmosphere by combusting a major portion of it. More specifically, determining the fuel flow parameter value based on the determined air flow related parameter value provides for a controlled supply of the excess gaseous fuel for the air and fuel premix which makes it possible to obtain combustion of the excess gaseous fuel which is considerably more complete than in known solutions. As little as only 2-5% of the excess gaseous fuel may remain un-combusted as a result of the second combustion mode. The reason is that the fuel flow related parameter value may be determined so as to provide premixed flame propagation in the cylinder, and therefore combustion of a major part of the excess gaseous fuel in the second cylinder. Thus, the invention may substantially reduce environmental disturbances caused by excess gaseous fuel in high pressure gas injection engines. Further, the invention may substantially increase the amount of useful work provided by the excess gaseous fuel, due to a higher portion of it being combusted in the engine.
Differing from the invention, adding, as suggested in said CA2868338A1, the premix of captured vented gaseous fuel and air before the regular direct gas injection, without any control of the fuel flow for the premix, may result in a major part of the fuel in the premix to exit the cylinder un-combusted. If the premix is lean, and the engine load is low, the combustion may be limited to a mixing controlled combustion of the regular direct gas injection and the premix locally involved in this combustion. However, in other parts of the combustion chamber the premix may not be combusted, and the fuel therein may be evacuated to the atmosphere.
The premixed flame propagation made possible by the invention is similar to the process in gasoline engines. However, determining the fuel flow related parameter value based on the air flow related parameter value makes it possible to obtain an air to fuel ratio providing a premixed flame propagation even if the engine is not equipped with a throttle for the air. Where there is no throttle, the air flow to the engine will depend largely on the engine rotational speed. Thus, the invention provides for adjusting the excess gaseous fuel supply to the detected air flow, and thereby secure a premixed flame propagation combustion. I.e., even though the engine is a “diesel type” HPGI engine, which typically has no throttle, an excess gaseous fuel and air pre-mixture may be provided which provides an “Otto type” premixed flame propagation combustion. Thereby, as opposed to the suggestion in said CA2868338A1, most of the excess gaseous fuel may be combusted even if the engine is in a low load condition. In some embodiments, the provision of the excess gaseous fuel is controlled so as to provide the premix for a load in the cylinder which is kept within an interval of 50-70% of a full load in the cylinder.
It should be noted that the second combustion mode may include, in addition to supplying the excess gaseous fuel to provide the premix, injection by means of the first gas injection system of the first gaseous fuel into the cylinder. For an unchanged load of the cylinder, such an injection of the first gaseous fuel in the second combustion mode is preferably smaller than the injection of the first gaseous fuel in the first combustion mode. As exemplified below, the first gaseous fuel injection in the second mode may serve as a pilot injection to initiate the premixed flame propagation combustion.
The first gas injection system may comprise a container, herein also referred to as a first container, the container of the second gas injection system herein also being referred to as a second container. The first container may be a storage container, e.g. a liquid natural gas (LNG) tank. The first gas injection system may also comprise a third container in the form of a high pressure buffer tank, arrange to be fed with the first gaseous fuel from the first container by means of a high pressure pump.
The second container, i.e. the container of the second gas injection system for accumulating the excess gaseous fuel, may be a small low-pressure gas accumulator. Upon accumulation the excess gaseous fuel may be on demand efficiently burned in the engine by said shifting from the first combustion mode to the second combustion mode. It is understood that the second combustion mode is dissimilar to the first combustion mode.
As exemplified below, the accumulation in the second container may be provided as a result of boil-off gas received from the first container, or upon transportation from the first gas injection system at a pressure decrease therein or during an engine stoppage. Upon such an accumulation, the gaseous fuel will be at a low pressure, and cannot be utilised as fuel in a normal HPGI combustion mode. Therefore, burning the second gaseous fuel in the second combustion mode, with a controlled excess gaseous fuel supply according to the invention, will provide an effective and controllable way to utilise this fuel.
Preferably, the premix has a lambda value of 1.3-1.7, more preferably 1.4-1.6, for example approximately 1.5. This secures a premixed flame propagation and therefore combustion of a major part of the excess gaseous fuel in the cylinder. Thereby, a minimisation of the excess gaseous fuel emissions can be obtained.
Preferably, the step of supplying the excess gaseous fuel comprises injecting the excess gaseous fuel into the cylinder or into a conduit arranged to guide air to the cylinder. For example, in the case of an engine with a plurality of cylinders, the second gas injection system may comprise one or more fuel injectors, and may be arranged to provide the excess gaseous fuel directly into one or more of the cylinders. As exemplified below, such an injection may be provided early enough for the injected excess gaseous fuel to mix with the air in the cylinder(s). Alternatively, the second gas injection system may be arrange to inject the excess gaseous fuel directly to one or more of air conduits dedicated only for the respective cylinder. Such dedicated conduits are also referred to as intake ports. Thereby, the excess gaseous fuel may be injected in the inlet port(s) and premixed with air prior to combustion. The second gas injection system being arranged to provide the excess gaseous fuel directly into one or more of the cylinders or directly to one or more of the air conduits dedicated only for the respective cylinder, allows for supplying the excess gaseous fuel to a selected subgroup of the cylinders only. As discussed below, this provides for an improved engine load control when emptying the container for accumulating the excess gaseous fuel. In alternative embodiments, the excess gaseous fuel is injected into an air conduit which is arranged to guide air to a plurality of, or all of the cylinders, of a multi-cylinder engine.
Preferably, the step of injecting the first gaseous fuel comprises injecting the first gaseous fuel at a first pressure, and the step of injecting the excess gaseous fuel comprises injecting the excess gaseous fuel at a second pressure which is lower than the first pressure. Where the first combustion mode is a high pressure gas injection (HPGI) combustion mode, the first pressure is relatively high. Contrary to this, the excess gaseous fuel is accumulated and injected at a relatively low pressure. It should be noted that the pressure of the injections during the first combustion mode may vary, e.g. based on the engine load. Thus, what is herein referred to as the first pressure may vary depending on the operational situation of the engine. In many embodiments, the first pressure is however in any operational situation preferably higher than the second pressure.
Preferably, the step of injecting the excess gaseous fuel comprises injecting the excess gaseous fuel before a crankshaft angle of 90 degrees before a top dead centre position at the end of a compression stroke of the cylinder. More preferably, the excess gaseous fuel is injected at the end of an induction stroke or at the beginning of the compression stroke of the cylinder. Thereby the excess gaseous fuel is injected early enough for a homogenous premix of air and excess gaseous fuel.
Preferably, the step of injecting the first gaseous fuel comprises injecting the first gaseous fuel after a crankshaft angle of 90 degrees before a top dead centre position at the end of a compression stroke of the cylinder. Where the first combustion mode is an HPGI combustion mode, the first gaseous fuel would be injected at the end of the compression stroke or at the beginning of an expansion stroke of the cylinder. It should be noted that preferably, the first combustion mode comprises no excess gaseous fuel supply.
Preferably, the second combustion mode comprises a pilot injection of a liquid fuel, such as diesel fuel. Thereby, the second combustion mode may provide a dual fuel combustion involving a premix of air and the excess gaseous fuel, and a diesel pilot ignition. The pilot injection is preferably controlled in dependence on the ratio of air and excess gaseous fuel in the premix. The liquid fuel pilot injection auto-ignites and provides thereby an initiation of the premixed flame propagation combustion. Preferably, the injection timing is similar as that of a pure diesel combustion mode.
Preferably, the step of injecting the first gaseous fuel comprises injecting the first gaseous fuel by means of a first injector, and the second combustion mode comprises a pilot injection of the first gaseous fuel by means of the first injector. This is particularly advantageous where the first gaseous fuel, and hence the excess gaseous fuel is natural gas. Natural gas puts higher requirements on the ignition system compared to engines fuelled with gasoline, since more energy from the spark is needed. The pilot injection of the first gaseous fuel assists the initiation of the premixed flame propagation combustion of the second combustion mode. This is particularly advantageous since it allows the use of a smaller capacity Diesel pilot injector than required for the total pilot fuel injection of the second combustion mode. More specifically, where the first combustion mode is an HPGI combustion mode, it may require a smaller amount of pilot fuel in each cycle than the second combustion mode. Using in the second combustion mode the first injector for a pilot injection of the first fuel may supplement a pilot injection of Diesel fuel. Thereby, the capacity of the Diesel pilot fuel injector does not need to be sized for the increased pilot fuel requirements of the second combustion mode. Thus, a relatively small capacity Diesel injector may be used, which is in the interest of cost control. It should be noted however, that in alternative embodiments, no first gaseous fuel is injected into the cylinder in the second combustion mode.
Preferably the method comprises, simultaneously to said step of supplying the excess gaseous fuel to provide a premix of air and the excess gaseous fuel to the cylinder, injecting, in the first combustion mode, by means of the first gas injection system, the first gaseous fuel into a further cylinder of the engine, the further cylinder being herein referred to as a first cylinder, and the cylinder in which the combustion mode is shifted from the first combustion mode to the second combustion mode being herein referred to as a second cylinder. It is understood that the second cylinder is not the first cylinder. It should be noted that the first gaseous fuel may be injected in the first combustion mode into one or more of the cylinders, but less than all cylinders. The excess gaseous fuel may be provided for the second combustion mode in one or more of the remaining cylinder, e.g. in only a single cylinder of the cylinders. It should be noted that in some embodiments, the simultaneous combustion modes in separate cylinders may be accomplished by deactivation of one or more of the remaining cylinders.
The simultaneous first and second combustion modes in separate cylinders allows for retaining effective and fully controllable high pressure injection combustion cycles in some of the cylinders, while the combustion of the excess gaseous fuel may be optimised in another cylinder or in other cylinders. The mode mix also makes it possible to equip only one or some of the cylinders with hardware for allowing the second combustion mode with the excess gaseous fuel, which is in the interest of cost control. Since the excess gaseous fuel is provided only in one, or in a subset of the cylinders, it may be secured that there is enough excess gaseous fuel for sustaining a controlled premix promoting premixed flame propagation combustion. This is particularly useful in a diesel type engine adapted for gaseous fuel, since there is no means to throttle the air provided to the engine. The concentration of the second combustion mode to only one, or a subset of the cylinders, may allow for a lambda value for premixed flame propagation combustion in the second combustion mode.
Preferably, the method comprises, before said step of shifting in the second cylinder from the first combustion mode to the second combustion mode, injecting (S1), in the first combustion mode, by means of the first gas injection system, the first gaseous fuel into all cylinders.
Preferably, the method comprises determining a required load of the engine, and controlling the injection, in the first combustion mode, of the first gaseous fuel into the first cylinder so as to provide a total load of the engine corresponding to the required load. Thereby, the simultaneous first combustion mode in the first cylinder(s) may be adjusted to allow optimization of the second combustion mode in the second cylinder(s) regardless of load requirements on the engine. More specifically, said step of supplying the excess gaseous fuel to provide a premix of air and the excess gaseous fuel to the second cylinder may be controlled so as to provide a lambda value of 1.3-1.7 for the premix, thereby securing premixed lame propagation in the second cylinder, and therefore combustion of a major part of the excess gaseous fuel in the second cylinder. Where there is no throttle on the engine, this may result in limited possibilities to control the load in the second cylinder. However, by controlling the injection, in the first combustion mode, of the first gaseous fuel into the first cylinder so as to provide a total load of the engine corresponding to the required load, both a possibility to meet varying load requirements and a minimisation of the excess gaseous fuel emissions can be obtained.
Preferably, the substantially constant load in the second cylinder corresponds to a load in an interval of 50-70%, preferably approximately 65%, of a full load in the second cylinder. Thus, the provision of the excess gaseous fuel may be controlled so as to provide a load in the second cylinder which is kept within an interval of 50-70% of a full load in the second cylinder, and simultaneously said injection of the first gaseous fuel in the first cylinder(s) may be controlled so as to provide a total load on the engine corresponding to the required load. Thereby, nearly all of the second fuel may be combusted, and at the same time the first combustion mode in the first cylinder(s) may be controlled so as to compensate for any discrepancy between the required load and the load in the second cylinder. For example, if the second cylinder is operating in the second mode at 65% of full load, and the required load is 50%, the first combustion mode can be controlled so as to provide less than 50% of full load in the first cylinder(s), so that the total load provided by the engine corresponds to the required load.
Preferably, the method comprises determining a pressure or gas content in the container, and performing said shift from the first combustion mode to the second combustion mode in dependence on the determined pressure or gas content in the container. In some embodiments, said shift is performed on the condition that the pressure or gas content in the second container is above a threshold value. Thereby, the shift may be performed when the accumulation in the container has reached the capacity of the second container. Also, the shift according to such embodiments may ensure that there is enough excess gaseous fuel in the container to allow the second combustion mode in a controlled manner.
Further advantages and advantageous features of the invention are disclosed in the following description.